When using a gear mate and mating a spur gear and a bevel gear with their faces at 90 deg why do they both rotate the same direction

SF 2015-06-05

Mating two bevel gears or two spur gears at 90 degrees result in the correct 'opposite' rotation of both gears. When mating a bevel and a spur at 90 degrees both rotate in the same direction.

See attached files

Solidworks Assemblies